1. Field
This invention pertains generally to support grids for nuclear fuel rods and more particularly to a holding fixture to assist in assembly of support grids for nuclear fuel rods.
2. Related Art
In most water cooled nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. In pressurized water nuclear reactors (PWR), these fuel assemblies typically include a plurality of fuel rods held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of elongated thimble tubes of the fuel assembly. The thimble tubes typically receive control rods or instrumentation therein. Top and bottom nozzles are on opposite ends of the fuel assembly and are secure to the ends of the thimble tubes that extend above and below the ends of the fuel rods.
The grids, as is known in the relevant art, are used to precisely maintain the spacing and support between the fuel rods in the reactor core, provide lateral support for the fuel rods and induce mixing of the coolant. One type of conventional grid design includes a plurality of interleaved straps that together form an egg-crate configuration having a plurality of roughly square cells which individually accept the fuel rods therein. Depending upon the configuration of the thimble tubes, the thimble tubes can either be received in the cells that are sized the same as those that receive fuel rods therein, or in relatively larger thimble cells defined in the interleaved straps. The interleaved straps provide attachment points to the thimble tubes, thus enabling their positioning at spaced locations along the length of the fuel assembly.
Referring to FIG. 1, a portion of an upper strap 2 and a lower strap 4 from a conventional grid design are shown. The straps 2,4 each include a plurality of slots 6. The slots 6 extend approximately half the height of the straps 2,4 and form tabs 7 beside each of the slots 6. The straps 2,4 are assembled by arranging the upper strap 2 perpendicular with respect to the lower strap 4 and sliding a slot 6 of the upper strap 2 into a corresponding slot 6 of the lower strap 4. While a portion of one upper strap 2 and one lower strap 4 are shown in FIG. 1, a conventional grid design typically includes twelve to sixteen sets of upper and lower straps 2,4. The upper and lower straps 2,4 may also include flow vanes 9 extending at an angle from the top portions of the upper and lower straps 2,4.
An example of a portion of an assembled conventional grid 10 is shown in FIG. 2 and an elevational view of a fuel assembly 40 employing the grid 10 is shown in FIG. 3. The flow vanes 9 are not shown in FIGS. 2 and 3. The fuel assembly 40 is of the type used in a pressurized water reactor and basically includes a lower end structure or bottom nozzle 42 for supporting the fuel assembly on a lower core plate (not shown) in the reactor core region and a number of longitudinally extending guide thimbles or tubes 44 which project upwardly from the bottom nozzle 42. The assembly 40 further includes a plurality of grids 10. The grids 10 are axially spaced along and supported by the guide thimbles 44. Assembly 40 also includes a plurality of elongated fuel rods 36 transversely spaced and supported in an organized array by the grids 10. Also, the assembly 40 has an instrumentation tube 46 located in the center thereof and an upper end structure or nozzle 48 attached to the upper ends of the guide thimbles 44. With such an arrangement of parts, the fuel assembly 40 forms an integral unit capable of being conveniently handled without damaging the assembly of parts.
As mentioned above, the fuel rods 36 and the array thereof in the assembly 40 are held in spaced relationship with one another by the grids 10 spaced along the fuel assembly length. Each fuel rod 36 includes nuclear fuel pellets 50 and the opposite ends of the rods 36 are enclosed by upper and lower end plugs 52 and 54, to hermetically seal the rod. Commonly, a plenum spring 56 is disposed between the upper end plug 52 and the pellets 50 to maintain the pellets in a tight, stacked relationship within the rod 36. The fuel pellets 50 composed of fissile material are responsible for creating the reactive power of the PWR. A liquid moderator/coolant, such as water or water-containing boron, is pumped upwardly through the fuel assemblies of the core in order to extract heat generated therein for the production of useful work.
To control the fission process, a number of control rods 58 are reciprocally movable in the guide thimbles 44 located at predetermined positions in some of the fuel assemblies 40. Specifically, the top nozzle 48 has associated therewith a rod cluster control mechanism 60, having an internally threaded cylindrical member 62 with a plurality of radially extending flukes or arms 64 such that the control mechanism 60 is operable to move the control rods 58 vertically in the guide thimbles 44 to thereby control the fission process in the fuel assembly 40, all in a well-known manner.
Assembling the grid 10 involves mating numerous upper and lower straps 2,4 together. However, the tight tolerances of the corresponding slots 6 in the upper and lower straps 2,4 make it difficult to properly align and mate the straps 2,4. In particular, it is difficult to automate the mating of the straps 2,4 and to mate multiple sets of straps 2,4 to each other simultaneously. As such, the assembly of the grid 10 is labor intensive, error prone and costly. It is thus desired to more efficiently assemble grids such as the conventional grid 10.